Energy absorbing landing gear

ABSTRACT

A landing gear for helicopters in which a member having a static spring rate and plastic yielding characteristics is mounted in series with a member having a static spring rate and a velocitysensitive restraining force. The combination serves to provide for appropriately &#39;&#39;&#39;&#39;soft&#39;&#39;&#39;&#39; landings under conditions of light gross weight and/or low vertical descent speed, and is also capable of absorbing the forces generated during &#39;&#39;&#39;&#39;hard&#39;&#39;&#39;&#39; or crash landings under conditions of heavy gross weight and/or high vertical descent speed. By designing the yield point of the plastic yielding member to a force approximately equal to the resistance offered by the velocity-sensitive unit at the time that is &#39;&#39;&#39;&#39;bottoms out,&#39;&#39;&#39;&#39; the efficient energy absorbing properties of the plastically yielding member are employed to extend the energy absorbing capabilities of the gear.

United States Patent [191 Fagan et a1.

[ Feb. 13, 1973 ENERGY ABSORBING LANDING GEAR [7 3] Assignee: Textron,Inc.

[22] Filed: June 11, 1970 [21] Appl. N0.: 45,363

[52] U.S. Cl ..244/108, 280/124 B, 280/124 F [51] Int. Cl ..B64c 25/52[58] Field-0f Search ..244/l08, 107, 104, 103, 102, 244/100, 17.17, 100R, 17.11; 188/1C;

[56] References Cited UNITED STATES PATENTS Primary ExaminerMiltonBuchler Assistant ExaminerPau1 E. Sauberer AttorneyRichards, I-Iarris &Hubbard 5 7] ABSTRACT A landing gear for helicopters in which a memberhaving a static spring rate and plastic yielding characteristics ismounted in series with a member having a static spring rate and avelocity-sensitive restraining force. The combination serves to providefor appropriately soft landings under conditions of light gross weightand/or low vertical descent speed, and is also capable of absorbing theforces generated during hard or crash landings under conditions of heavygross weight and/or high vertical descent speed. By designing the yieldpoint of the plastic yielding member to a force approximately equal tothe resistance offered by the velocity-sensitive unit at the time thatis bottoms out, the efficient energy absorbing properties of theplastically yielding member are employed to extend the energy absorbingcapabilities of the gear.

15 Claims, 6 Drawing Figures PAIENTEDFEBIISIQYS 3,716,208

' SHEET 10F 3 .N IEHTORS.

CASTLE H. FAGAN ROBERT R. LYNN ATTORNEYS PA'TENTEDFEBIBIBTS 3.716.208sum 2 or a VENTOR ROBERT R LYNN ATTORNEYS LOAD ON GEAR IKIPS.)

PAIENTED FEB I 3 I975 SIIEEI 3 BF 3 LANDING GEAR ENERGY ABSORPTIONPERMANENT EROSS TUBE LIQUID SPRING ONLYW l DEFLECTION OF CROSS BARS PLUSMOVEMENT OF LIQUID SPRING SHOCKS SUM OF ENERGIES FROM LIQUID SPRINGS ANDCROSS TUBES I I I I I I I I I O I 2 3 4 5 6 7 8 9 I0 I06 -INCHES-AIRCRAFT TRAVEL AFTER TOUCH'DOWN INVENTORS'. 6 CASTLE H. FAGAN ROBERT R.LYNN ATTORNEYS ENERGY ABSORBING LANDING GEAR BACKGROUND OF THE INVENTIONSkid type helicopter landing gears, such as that set forth in U. S. Pat.No. 2,641,423, perform their function of energy storing and absorptionthrough the deflection of a member that has a static spring rate, i.e.,the load development is proportional to the deflection of the memberwhich is designed to retain its normal shape under the loads produced bycrash or hard landings. Under normal loads, where the member will bendbut will not plastically deform, the energy is stored in the member byits deflection (a small portion of it being dissipated by heat producedin the bending process) and is then fed back into the aircraft, oftenresulting in a bouncing type of landing. Under hard or crash landingconditions, the energy is dissipated, rather than stored, through thepermanent plastic deformation of the member. In order to provide alanding gear of this type that can accommodate hard or crash landingswithout damage to the aircraft structure, this member must be capable ofabsorbing a large amount of energy and thus will be hard or stiff, aquality that makes for a very rigid structure in the context of anordinary landing and consequently fails to provide a comfortable gearfor the ordinary landing. One way of appreciating the problem is toassume two landings of the same helicopter at the same rate of descent,one landing with the aircraft at a high gross weight and the otherlanding with the aircraft at a low gross weight. In the case of the highgross weight landing, in order to absorb or store the energy, the gearmust be of a certain stiffness and must deflect a certain amount. Theforce required to deflect the member per unit of deflection multipliedby the units deflected equals the energy present in the descendingaircraft that must be absorbed by the gear. Assuming that the gear isdesigned to accommodate the condition of the high gross weight landing,it can be appreciated that the gear will offer an uncomfortable, bouncyfeel to the pilot and occupants of the aircraft under low gross weightlanding conditions. For example, if the low gross weight of the aircraftis 65 percent of the high gross weight, then in order to absorb theenergy of descent during the low gross weight landing the gear willdeflect about 65 percent of that which it would have deflected at thehigh gross weight. This means that the pilot and occupants will bedecelerated over only 65 percent of the distance and they will undergohigher, and uncomfortable, g landings. To the pilot and occupants, theexperience will be that of a rough landing. Considering the wideextremes of weights and landing speeds that one wishes to accommodate inthe same helicopter, it should be clear that the conventional skid gearcannot be readily adapted to perform suitable function for allconditions of loading and/or operation of the modern helicopter.

Oleo struts, liquid spring gears and the like do not plastically yield,but operate with static spring rates and velocity-sensitive restrainingforces. By virtue of the velocity-sensitive restraining forcecharacteristic, the harder the landing the more energy absorbingcapability the unit has, and a landing gear of this type can morereadily be designed to accommodate a wider range of aircraft grossweights than can the previously noted type.

However, while the energy absorbing capability of the liquid spring gearand the oleo strut type of landing gear is proportional to the landing(vertical) velocity and thus is adaptable to a wider range of grossweights than the conventional skid gear, its ultimate energy absorbingcapability, considering only a crash landing, may not be as great as theenergy absorbing capability inherent in the skid landing gear noted inthe aforenoted U. S. Pat. No. 2,641,423 in which, in the event of acrash, the cross tubes will permanently yield and rupture. Yielding andrupture of metal is a more efficient method of energy absportion thandissipation of energy through heat transmission as takes place in theoleo strut and liquid spring.

It should be appreciated that in the event of landings that may beclassified as crash landings, the primary concern is that ofpreservation of life and limb, rather than that of comfort and it istherefore of the utmost importance that the energy absorbingcharacteristics of the gear be as efficient as possible under a crashlanding, absorbing the greatest amount of energy as quickly as possible.

The present invention serves the purpose of providing a landing gearthat has comfortable soft and moderate landing characteristics whileextending energy absorption capability and efficiency under crashlandings, and it achieves this by mounting in series a member withstatic spring rate and plastic yielding characteristics with a memberthat has a static spring force and offers a velocity-sensitiverestraining force.

DESCRIPTION OF THE DRAWINGS For a more complete understanding of thepresent invention and for further objects and advantages thereof,reference may now be had to the following description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a fragmentary view of one side of a helicopter showing onelanding skid mounted -in accordance with the present invention;

FIG. 2 is a detailed showing of the flange used for coupling the aftcross tube to the fuselage;

FIG. 3 is a side view of the aft cross tube mounting showing the'tube insection;

FIG. 4 illustrates the forward cross tube mounting;

FIG. 5 is a side view partially in section showing the coupling betweenthe cross tubes and the skids by way of liquid spring shocks; and

FIG. 6 is an energy absorption diagram for the landing gear of thepresent invention.

DETAILED DESCRIPTION In FIG. 1, the left side landing gear for ahelicopter 10 has been illustrated. A skid 11 of conventional type isemployed to provide the ground contact for the landing gear. The skid 11is coupled to the fuselage by way of a forward cross tube 12 and a rearcross tube 13. The forward cross tube 12 pivots whereas the rear crosstube 13 is fixed as will hereinafter be further detailed with both crosstubes coupled to the skid 11 by way of liquid spring shocks. By liquidspring shock there is meant a liquid filled hydraulic cylinder unit withan internal bypass therein from one side of the piston to the other.

As shown, the end of the forward cross tube 12 is connected to the skid11 by way of a liquid spring shock unit 14 whereas the aft cross tube 13is connected to the rear of the skid 11 by way of a liquid spring shockunit 15. Of course, a similar skid is supported on the right side of thehelicopter by similar cross tubes, liquid spring shock units, etc.

FIGS. 2 and 3 illustrate one form in which the aft cross tube is lockedinto a coupling 20. FIG. 2 is an isometric view partially in section toshow the aft cross tube extending through a mounting trunnion. Moreparticularly, the fuselage is provided with a bracket 30 which isprovided with vertical webs 31 and 32 and a horizontal web 33. A thinvertical panel 34 extends between webs 31, 32 and 33. a thick verticalpanel 34a extends between webs 31 and 32 below the web 33 and isprovided with a generally circular opening 34b in which a trunion 35 isreceived.

Trunion 35, FIG. 3, has a pair of trunion lugs 36 and 37 which fit intonests in the web 34a. On the left side of FIG. 3 a pair of holes 38 and39 are provided for clamping retaining straps which retain the lug 37 inthe web 34a. On the right hand side, the strap 40 is shown in placebeing secured by bolts 41 and 42.

The aft cross tube 13 is shown in section secured to the trunion 35 bymeans of two cross pins 44 and 45. The pin 44 extends through thetrunnion 35 and also pin 45 extends through inboard of pin 44. The pins44 and 45 key the cross tube 13 to the trunnion 35 to prevent rotationthereof. The trunion lugs 36 and 37 couple the trunion to the bracket 30so that the cross tube 13 is prevented from rotation relative to thefuselage. A small gap is provided between the lugs and the retainingstraps such as would be between lug 36 and strap 40. The gap allowslateral motion of lugs 36 and 37 during deflection.

In contrast, FIG. 4 illustrates the mounting for the forward cross tube12. A pad 50 is secured by rivets 51 to the tube 12. The pad 50 isprovided with exterior arcuate rib sections which maintain thelongitudinal position of the cross tube 12 relative to the fuselage butpermits rotation about a horizontal axis. The fuselage carries a bearingplate to mate with pad 50 to transfer weight of the aircraft onto theforward cross tube.

FIG. 5 illustrates the coupling used for mounting the liquid springshocks between the ends of each of the forward and aft cross tubes andthe skid. The aft cross tube 13 is shown coupled at its extremitythrough a pivot 60 to a crank 61. The crank 61 is mounted on skid 11 bya pivot 62 and is pivotally coupled to the end 63 of a piston rod 64 andpiston 64a which works in and is part of a liquid spring shock cylindera for shock 15. Cylinder 150 is mounted at pivot 65 near the rear end ofthe skid 11.

The liquid spring shocks of the type manufactured and sold by TaylorDevices, North Tonawanda, N.Y., have been found to be satisfactory. Suchunits are those which in one embodiment of the invention provided a 3.5inch liquid spring stroke for an 8 inch travel of the pivot pin 60. Theactuation of this device resulted in a static spring force and avelocity spring force being applied to the cross tube 13. The returnstroke of the unit acts as a damper which dissipates the stored energyand prevents excessive rebound. The fluid used in the above units was ofthe type manufactured by Dow Corning and identified as Dow Corning 200.Such fluid is available in viscosities from 0.65 to 60,000 centistokes.It was preferred that, for use with a helicopter of the typemanufactured and sold by Bell Helicopter Company, Fort Worth, Tex., andidentified as 205A Helicopter, the viscosities were l,000 and 350centistrokes for the aft and forward shocks, respectively. The use ofsuch liquid spring shocks with such fluid provided an optimumcharacteristics for the above aircraft; however, it is noted that othertuned energy absorbing devices which have both a static and velocityspring force capability could be used.

As shown in FIG. 6, the landing gear energy absorption for a particularlanding is plotted, the curve representing the sum of energies absorbedby the liquid springs and cross tubes. It will be noted that in theregion from zero inches travel (travel meaning vertical travel of theaircraft after touchdown through deflection or movement of the gear) toabout 6.25 inches travel, the deflection of the cross beams and themovement of the liquid spring shocks combine to give a substantiallyconstant load-deflection characteristic. For lower gross weights orlower velocity sink characteristics the slope of this line would belesser since the velocity spring rate of the liquid spring shock wouldthen be lower. After about 6.25 inches of travel, the energy is absorbedonly in the liquid spring which, through a proper relationship betweenthe velocity spring rate and the static spring rate of the springs, isdesigned to then maintain a relatively constant force until it bottomsout. The liquid spring was designed to bottom out shortly after about10.6 inches of travel, at which time the cross tube begins to deformplastically and act to further absorb the energy of the crash" in ahighly efficient manner. As shown by the dotted line, the load continuessubstantially constant through the plastic deformation of the crosstubes.

Provided that the landing is not so severe that the fuselage strikes theground, the type of gear as presented tends to make the landingrelatively soft because the load is maintained substantially constant.Also, as previously stated, a large amount of energy is absorbed by thetype of gear herein presented, through the combining of characteristicsof the cross tubes with those of the liquid spring shocks.

Thus, by combining certain characteristics of the cross tubes withcertain characteristics of the liquid spring shocks, a desirable energyabsorption function is built into the landing gear and prevents undulyhard landings and protects the landing gear in all normal operations aswell as in abnormal operations. As has been stated, forces involved inthe landing are minimized by providing protection for the cross tubesthrough the use of the spring shocks up to the point that the springshocks have been deflected through their entire range. Thereafter, thepermanent set taken in the cross tubes at load levels which thepersonnel and the aircraft involved can withstand.

It will be understood that a static spring force is created by forcingthe piston rod 64 into the cylinder 15a. This reduces the volume andcompresses the liquid. Thus, the static spring force is a function ofthe preload pressure and the piston rod position. This spring constantcan be changed with the change in piston rod diameter and/or by using afluid with higher compression spring rate. Also, a preload pressurechange can be used to change the initial static spring force resultingin a change in the amount of static energy stored. The velocity springforce, on the other hand, due to the use of the liquid spring shock is aresult of the resistance created as the fluid is forced past the pistonend 64a at a high rate. Thus, the greater the piston velocity, thehigher the force. As above noted,

because the fluid must also flow past the piston on the I return stroke,the unit acts as a damper and dissipates the stored energy. The velocityspring characteristics can be changed by modifying the unit to changethe cylinder to piston clearance or by using the fluid that has adifferent viscosity. Therefore, a wide range of energy absorptioncharacteristics can be achieved by the minor changes after the unit isfabricated.

It has been found that the efficiency of the device can be approximately90 percent. Efficiency is defined as the energy absorbed in inch poundsdivided by the sum of the maximum load (in pounds) times the distancetraveled (in inches). However, the system operating at approximately 80percent efficiency was found to be an optimum level for good landingcharacteristics throughout the gross weight range of the aircraft. Inthe drawings shown in FIG. 6, the aircraft travel after touchdown was10.6 inches. However, the unit illustrated actually will allow 12 inchesof travel of the center of gravity. The efficiency of the landing gearis approximately 70 percent for a 9,000 pound gross weight vehicle and asink speed of about 8 feet per second. Higher values will be obtainedwith a higher gross weight aircraft and/or higher sink speeds.

Although a specific embodiment of the invention is illustrated in thedrawings and described herein, it will be understood that the inventionis not limited to the embodiment disclosed but is capable ofrearrangement, modification and substitution of parts and elementswithout departing from the spirit of the invention.

What is claimed is:

1. An energy absorbing landing gear for an aircraft comprising:

a landing skid;

an elongated tubular spring member with one end connected to theaircraft;

crank means with one end rotatably attached to said landing skid and theother end of the crank means rotatably attached to the other end of saidtubular spring; and

a liquid spring with one end rotatably connected to said crank and theother end rotatably connected to said skid.

2. An energy absorbing landing gear as defined in claim 1 wherein:

said tubular spring member deflects upon the application of normal loadsand deforms plastically upon the application of excessive loads; and

said liquid spring member provides velocity sensitive resistance to theapplication of loads.

3. An energy absorbing landing gear for an aircraft comprising:

a landing skid;

an elongated tubular spring member with one end connected to theaircraft;

crank means rotatably attached to the other end of said tubular spring;and

a liquid spring with one end rotatably connected to said crank and theother end rotatably connected to said skid, said crank means beingrotatably connected to said skid at a point on said crank 5 between thepoints said tubular and liquid springs connect to said crank.

4. An energy absorbing landing gear as defined in claim 3 wherein:

said tubular spring member deflects upon the application of normal loadsand deforms plastically upon the application of excessive loads; and

said liquid spring member provides velocity sensitive resistance to theapplication of loads.

15 5. An energy absorbing landing gear for an aircraft comprising:

a landing skid;

a pair of tubular springs with one end connected to the aircraft;

a pair of crank means with a first end of each crank means rotatablyattached to one of each of the other ends of said tubular springs and asecond end of each crank means rotatably attached to said landing skid;and

a pair of liquid springs each with one end rotatably connected to one ofsaid cranks and the other end rotatably connected to said skid.

6. The energy absorbing landing gear according to claim 5 wherein theliquid springs bottom upon the application of a predetermined loadbetween the landing skid and the aircraft and wherein the tubularsprings begin to collapse in plastic deformation upon the application ofsubstantially the same predetermined load.

7. An energy absorbing landing gear for an aircraft comprising:

a landing skid;

a pair of tubular springs with one end connected to the aircraft;

a pair of crank means with a first end of each of said crank meansrotatably attached to one of each of the other ends of said tubularsprings and a second end of each crank means rotatably attached to saidlanding skid;

a pair of liquid springs each with one end rotatably connected to one ofsaid cranks and the other end rotatably connected to said skid, saidcrank means being connected to said skids at apoint on said crankbetween the points said tubular and liquid springs connect to saidcrank.

B. The energy absorbing landing gear according to claim 7 wherein theliquid springs bottom upon the application of a predetermined loadbetween the landing skid and the aircraft and wherein the tubularsprings begin to collapse in plastic deformation upon the application ofsubstantially the same predetermined load.

9. An energy absorbing landing gear for an aircraft as defined in claim7 wherein:

said tubular and liquid springs are adapted whereby said springs arecooperable in a first low load range to linearly deflect with load, saidsprings operable in a second higher load range wherein said first springis fully deflected whereby said second liquid spring absorbssubstantially all energy, and operable at a third highest load rangebeyond the range of said liquid spring wherein said first springplastically deforms to absorb energy.

10. An energy absorbing landing gear for an aircraft as defined in claim7 wherein:

said pair of tubular springs are located at forward and rear positionsof the aircraft with said forward tubular spring being mounted forrotation relative to the aircraft and said rear tubular spring is fixedrelative to the aircraft.

11. The energy absorbing landing gear according to claim 1 wherein themovement of the liquid spring member is limited by a stop, wherein theliquid spring member engages the stop upon the application of apredetermined load, and wherein the tubular spring member begins toplastically deform upon the application of substantially the samepredetermined load.

12. The energy absorbing landing gear according to claim 1 wherein thetubular spring member is connected to one point on said crank andwherein the liquid spring member is connected to another point on saidcrank.

13. The energy absorbing landing gear according to claim 1 wherein theliquid spring shock functions both as a load dependent spring and as aviscous damper.

14. An energy absorbing landing gear for an aircraft as defined in claim5 wherein:

said tubular and liquid springs are adapted whereby said springs arecooperable in a first low load range to linearly deflect with load, saidsprings operable in a second higher load range wherein said first springis fully deflected whereby said second liquid spring absorbssubstantially all energy, and operable at a third highest load rangebeyond the range of said liquid spring wherein said first springplastically deforms to absorb energy. 15. An energy absorbing landinggear for an aircraft as defined in claim 5 wherein:

said pair of tubular springs are located at forward and rear positionsof the aircraft with said forward tubular spring being mounted forrotation relative to the aircraft and said rear tubular spring is fixedrelative to the aircraft.

1. An energy absorbing landing gear for an aircraft comprising: alanding skid; an elongated tubular spring member with one end connectedto the aircraft; crank means with one end rotatably attached to saidlanding skid and the other end of the crank means rotatably attached tothe other end of said tubular spring; and a liquid spring with one endrotatably connected to said crank and the other end rotatably connectedto said skid.
 1. An energy absorbing landing gear for an aircraftcomprising: a landing skid; an elongated tubular spring member with oneend connected to the aircraft; crank means with one end rotatablyattached to said landing skid and the other end of the crank meansrotatably attached to the other end of said tubular spring; and a liquidspring with one end rotatably connected to said crank and the other endrotatably connected to said skid.
 2. An energy absorbing landing gear asdefined in claim 1 wherein: said tubular spring member deflects upon theapplication of normal loads and deforms plastically upon the applicationof excessive loads; and said liquid spring member provides velocitysensitive resistance to the application of loads.
 3. An energy absorbinglanding gear for an aircraft comprising: a landing skid; an elongatedtubular spring member with one end connected to the aircraft; crankmeans rotatably attached to the other end of said tubular spring; and aliquid spring with one end rotatably connected to said crank and theother end rotatably connected to said skid, said crank means beingrotatably connected to said skid at a point on said crank between thepoints said tubular and liquid springs connect to said crank.
 4. Anenergy absorbing landing gear as defined in claim 3 wherein: saidtubular spring member deflects upon the application of normal loads anddeforms plastically upon the application of excessive loads; and saidliquid spring member provides velocity sensitive resistance to theapplication of loads.
 5. An energy absorbing landing gear for anaircraft comprising: a landing skid; a pair of tubular springs with oneend connected to the aircraft; a pair of crank means with a first end ofeach crank means rotatably attached to one of each of the other ends ofsaid tubular springs and a second end of each crank means rotatablyattached to said landing skid; and a pair of liquid springs each withone end rotatably connected to one of said cranks and the other endrotatably connected to said skid.
 6. The energy absorbing landing gearaccording to claim 5 wherein the liquid springs bottom upon theapplication of a predetermined load between the landing skid and theaircraft and wherein the tubular springs begin to collapse in plasticdeformation upon the application of substantially the same predeterminedload.
 7. An energy absorbing landing gear for an aircraft comprising: alanding skid; a pair of tubular springs with one end connected to theaircraft; a pair of crank means with a first end of each of said crankmeans rotatably attached to oNe of each of the other ends of saidtubular springs and a second end of each crank means rotatably attachedto said landing skid; a pair of liquid springs each with one endrotatably connected to one of said cranks and the other end rotatablyconnected to said skid, said crank means being connected to said skidsat a point on said crank between the points said tubular and liquidsprings connect to said crank.
 8. The energy absorbing landing gearaccording to claim 7 wherein the liquid springs bottom upon theapplication of a predetermined load between the landing skid and theaircraft and wherein the tubular springs begin to collapse in plasticdeformation upon the application of substantially the same predeterminedload.
 9. An energy absorbing landing gear for an aircraft as defined inclaim 7 wherein: said tubular and liquid springs are adapted wherebysaid springs are cooperable in a first low load range to linearlydeflect with load, said springs operable in a second higher load rangewherein said first spring is fully deflected whereby said second liquidspring absorbs substantially all energy, and operable at a third highestload range beyond the range of said liquid spring wherein said firstspring plastically deforms to absorb energy.
 10. An energy absorbinglanding gear for an aircraft as defined in claim 7 wherein: said pair oftubular springs are located at forward and rear positions of theaircraft with said forward tubular spring being mounted for rotationrelative to the aircraft and said rear tubular spring is fixed relativeto the aircraft.
 11. The energy absorbing landing gear according toclaim 1 wherein the movement of the liquid spring member is limited by astop, wherein the liquid spring member engages the stop upon theapplication of a predetermined load, and wherein the tubular springmember begins to plastically deform upon the application ofsubstantially the same predetermined load.
 12. The energy absorbinglanding gear according to claim 1 wherein the tubular spring member isconnected to one point on said crank and wherein the liquid springmember is connected to another point on said crank.
 13. The energyabsorbing landing gear according to claim 1 wherein the liquid springshock functions both as a load dependent spring and as a viscous damper.14. An energy absorbing landing gear for an aircraft as defined in claim5 wherein: said tubular and liquid springs are adapted whereby saidsprings are cooperable in a first low load range to linearly deflectwith load, said springs operable in a second higher load range whereinsaid first spring is fully deflected whereby said second liquid springabsorbs substantially all energy, and operable at a third highest loadrange beyond the range of said liquid spring wherein said first springplastically deforms to absorb energy.